In recent years, industrial and automobile motors and industrial and automobile inverters have been rapidly downsized and increased in output, and insulating materials are demanded to have considerably superior properties. Particularly, the amount of heat generated from conductors densified in association with downsizing has been increased, making dissipation of the heat an important problem. As insulating materials used in these motors and inverters, cured articles containing a thermosetting resin are widely employed because of their high insulation performance, ease of molding, heat resistance and the like. However, since thermosetting resin cured articles generally have a low thermal conductivity and are thus a major factor that interferes with heat dissipation, it is desired to develop a resin cured article having a high thermal conductivity.
As resin cured articles having a high thermal conductivity, cured articles of epoxy resin compositions having a mesogen skeleton in the molecular structure have been proposed (see, for example, Patent Document 1). In Patent Document 1, it is disclosed that the thermal conductivity is improved by introducing a rigid structure, which is represented by a so-called mesogen skeleton, into the molecules of a resin and utilizing intermolecular stacking to allow expression of liquid crystallinity or crystallinity and to thereby suppress phonon scattering. As epoxy resins having a mesogen skeleton in the molecular structure, those compounds described in Patent Documents 2 to 4 and the like are exemplified.
For achieving an increase in the thermal conductivity of a resin cured article, there is a method of preparing a composite material by incorporating a thermal conductive filler composed of inorganic ceramic powder into a resin composition. As the thermal conductive filler, for example, alumina, magnesium oxide, beryllium oxide, boron nitride, aluminum nitride, silicon nitride, silicon carbide, aluminum fluoride, and calcium fluoride are known. This method is aimed at attaining both high thermal conductivity and electrical insulation in a composite material by incorporating a filler that exhibits both high thermal conductivity and electrical insulation into a resin composition.